/**
 * Method which inserts a full background-image property for a theme image.
 * It checks if the file exists and if it doesn't, it'll throw an error. 
 * By default it will not include the background-image property if it is not found,
 * but this can be changed by changing the default value of $include-missing-images to 
 * be true. 
 */
@function theme-background-image($theme-name, $path, $without-url: false, $relative: false) {
    $exists_image: theme-image($theme-name, $path, true, false);
    
    @if $exists_image {
        $exists: theme_image_exists($exists_image);

        @if $exists == true {
            @return theme-image($theme-name, $path, $without-url, $relative);
        }
        @else {
            @warn "@theme-background-image: Theme image not found: #{$exists_image}";

            @if $include-missing-images {
                @return theme-image($theme-name, $path, $without-url, $relative);
            }
        }
    }
    @else {
        @warn "@theme-background-image: No arguments passed";
    }
}